With the explosive growth of mobile data demand, future wireless networks would exploit new available frequency spectra, i.e., centimeter to millimeter wave (mmWave) brands, to greatly increase communication capacity. The fundamental differences between mmWave communication systems and the existing micro-wave systems operating below 5 GHz are high propagation loss, directivity, and sensitivity to blockage. To address this issue, directional beamforming with very high gain generated by an antenna array containing a large number of antenna elements has been considered as an essential technique to improve the signal strength level at the receiver. Because of the highly directional antenna employed at the BS or User Equipment (UE), beam or direction scanning at the BS and/or UE is required to achieve seamless coverage like the conventional cellular network. In addition, to further increase the system capacity, multiple antenna arrays are used at the BS and/or UE so that MIMO technique even Multi-User MIMO (MU-MIMO) technique can be employed in mmWave communication systems. One of the most important techniques to realize MIMO and MU-MIMO in mmWave communication systems is hybrid beamforming (HB), which is the combination of analog beamforming for Radio Frequency (RF) signals and precoding for digital or baseband signals that can be employed at both the BS and UE. The adaptive analog beamforming can complete beam/direction scanning with a high gain beam while the precoding can remove the interferences among the multiplexed data streams in MIMO and/or MU-MIMO.
Because of the small size of mmWave antenna array, it is may be integrated with the RF circuits, which means that there are no antenna ports for using cables to connect the BS RF paths to the channel emulator as the conventional method dose when testing the BS. Moreover, connecting each antenna element of the BS to the corresponding antenna element of the channel emulator directly with a waveguide or other similar techniques is still impossible as the size of each antenna element and the space between any two antenna elements are two small to place these huge number of waveguides. Other methods like passing the digital signals before digital-to-analog converter (DAC) at the BS to the channel emulator directly still faces a problem, i.e., the adaptive analog beamforming at the BS and the corresponding beam scanning cannot be emulated at the emulator. In summary, there are no prior art hardware RF channel emulator exits that meet these needs. This invention presents circuits and methods for building a hardware RF channel emulator that meet the needs of testing of mmWave communication systems.